1. Field of the Invention
This invention relates generally to the field of lifts and more particularly hoists and suspension systems based on pulleys and cables that are stabilized against tilting and rolling, such as those for lifting, suspending and lowering watercraft.
2. Description of Related Art
Mechanisms for lifting and lowering large loads based on pulleys and cables are especially useful because they are light, strong, and adaptable to many loads and easily powered by one or more motors or a manually driven winch. The alternatives to cables and pulleys usually involve hydraulics, levers, guide channels, screws or other complex mechanisms that add to the cost and complexity and reduce the reliability.
Lifts that remove and replace recreational power vessels and sailing vessels have special requirements. They need to be adaptable to a range of vessels, often over fifty feet in length and up to 50,000 or 70,000 pounds in weight. Simplicity of design, low cost and economy of maintenance are paramount. Moreover, for watercraft it is often inconvenient or practically impossible to provide for cables or structural members that pass overhead across the vessel.
In addition, it is necessary in most applications to lift and lower the load without allowing it to rotate about a horizontal axis. For vessel loads, this is equivalent to the requirement that the vessel be stable against forces that would produce roll or pitch. Stability is usually achieved by employing separate cable lines at the four corners of a rectangular lift mechanism, each separately powered, or powered in pairs from two electrical motors attached to long cylindrical pipes extending along the length of the vessel on either side.
The four-motor lift leaves access to the vessel unobstructed, because each one of the motors may be mounted on one of four posts, which are separated fore-aft and starboard-port so that they do not seriously impede access to the vessel. It has the obvious disadvantage of requiring more than the minimum number of motors. The two-motor lift, on the other hand, has two top rails, or beams, and two parallel pipes to transmit power fore-aft. The attendant top rails not only add to the cost of the lift but they also impede access to the vessel for repair, cleaning, loading and boarding. Moreover, the location of one of the electrical motors outboard of the dock presents additional problems of servicing a motor without dock access (unless additional dockage is provided) and maintaining electrical power cables under water.
In view of the foregoing, it is evident that a need exists for a simple mechanism, based only on cables and pulleys, that lifts and lowers large loads without rotation, and without the use of cables or structural members that pass over the load, with a single electrical motor, located on the dockside, and without rails or other members that impede access to the vessel. Several patents have attempted to address these needs.
U.S. Pat. No. 4,401,335, Golbersen, is directed to a boat hoist including a lift suspension system which results in even application of forces to points on the moveable platform remote from the point of attachment of the lifting device, thus eliminating twisting of the loaded platform.
This patent achieves equal loading of lift points by means of a cable arrangement similar to that of U.S. Pat. No. 5,427,471, which requires that one end of each of two cables be fixed below the surface of the water, and suffers from that same disadvantage.
U.S. Pat. No. 5,090,841, Penick, Jr. et al., is directed to a boat lift in which a hydraulic pump and cylinder with a piston is mounted on a manual boat lift such that the piston rod carries a pulley which engages the lifting cable so that when the piston moves in the cylinder the boat will move up or down.
This patent requires that part of the cable be immersed in the water while the boat is in the lifted position, which has the same disadvantages as U.S. Pat. No. 5,427,471. Moreover the invention calls for a piston and cylinder in addition to lines and pulleys, considerably adding to the maintenance costs of the system.
U.S. Pat. No. 5,427,471, Golbersen, is directed to a boat lift, including a vertical main post mounted on dock with the lower end mounted in the water but spaced above the floor of the water. A frame includes an outer guide post having a platform mounted on the water floor with an upper end above the water level; the frame is connected by a single member to the main post. A platform unit for supporting a small boat is slidably connected at both ends to the main post and the guide post, and a cable and winch unit is interconnected among the foregoing elements for effecting a level lifting and lowering of the platform unit into and out of the water.
This patent suffers from the defect that it requires one end of the cable to be secured at the bottom of the main post, at which point it is continually immersed in the water. This requires frequent replacement of the cable, or the use of a non-corrosive cable material, which adds considerably to the cost of installation and maintenance. Also, because of the restricted size of the vessels that can be lifted by this device, use on large multi-hull vessels is impractical.
U.S. Pat. No. 5,769,568, Parkins et al., is typical of current designs. It consists of two motors, two top rails and four winders operating in a configuration as previously described. It has all the disadvantages of currently employed lifts, namely more than one motor and two top rails.
U.S. Pat. No. 5,772,360, Wood II, has two motors, no top rail and two beams passing under the load. The dockside and water side of each beam are raised and lowered by separate cables, each wound on spools driven by the motor on the dockside pile. These two lines have mechanical advantages of one and three, the different mechanical advantages being compensated for by differential drives on the motor. This arrangement has no top rails and two motors on the dockside, but requires a differential gearing system.
U.S. Pat. No. 5,957,623, Sargent et al., has no top rail and no motor. It consists of a nut on a threaded vertical shaft which nut is rotated to raise or lower a boat attached to the nut.
U.S. Pat. No. 6,470,816, Golden et al., in a four post/dual motor embodiment, raises and lowers the vessel on two beams, one fore and one aft of the vessel, using two motors on the dock side and no top beam. This invention, however suffers from the fact that it is not stable to roll. Further, it requires that the cables be permanently secured below the water, which greatly shortens their life.
U.S. Pat. No. 6,408,776, McLaughlin et al., and U.S. Pat. No. 6,640,736, McLaughlin et al., consists of two beams under the vessel in the shape of a “V.” The lift is powered by a single motor at the top of the dockside post, spooling two lines that are each reeved through pulleys to a waterside post with a mechanical advantage of three, and also spooling two other lines each connected to the dockside of one of the beams with a mechanical advantage of one. The two different mechanical advantages are offset by drums of different diameters. The two beams and the vessel are raised or lowered synchronously depending on the direction of rotation of the motor.
These patents have no top rail and only a single dockside motor. The stability of the lift, however, is limited by the “V” shape of the beams, since it presents an asymmetrical support for the vessel. Further, many vessels are heavier at the stern and must be supported there. The lift is unsuitable for catamarans, which must be supported at four symmetrically located coplanar points. Finally, the inventions require two different diameter winches, which adds to the cost and complexity.
An earlier version of these inventions, U.S. Pat. No. 6,230,639, McLaughlin et al., employed two separate waterside posts. In this earlier version, the stability problem is solved, but as in the case of the 2002 and 2003 patents, the cost and complexity of the lift is increased by the use of two winders of different diameters.
U.S. Pat. No. 6,484,655, Gibson, has one motor and no top rail. It has two beams in a “V” formation, similar to other three-piling lifts. It is subject to the limitation of the other three-piling lifts and in addition it is unstable to rolling.
U.S. Pat. No. 7,070,171, Mangin, is directed to a suspension and hoisting system including two cable circuits, each including a cable fixed at one end and attached to a lifting apparatus, such as a winch, at the other, and being reeved around deflection pulleys mounted on two parallel shafts located at either end of the load support, and with the lifting apparatus of each cable being mounted above the load support on the side of the load that is opposite its fixed end. The cable of at least one of the cable circuits is reeved successively around at least two coaxial deflection pulleys mounted on each end shaft in such a way that on at least one of the shafts at least one pulley of one cable circuit rotates in the same direction as at least one pulley of the other cable circuit under identical lifting and lowering action of the two lifting apparatus, while these two pulleys are constrained to rotate in opposite directions by any downwardly oriented force independent of the action of the lifting apparatus. Accordingly, these two pulleys are interlocked to reduce or eliminate any tendency of the load support to incline under the effect of this downward force.
This patent suffers from reliance on friction between cable and pulley to prevent the downward force from lowering one end of the load. In many applications, maintaining a friction-free contact between pulley and cable is difficult because of the presence of water, grease, and dirt on the cables. Moreover, the two cable circuits are of unequal length, which results in unequal elongation under temperature variations, which will tend to make one or the other cable prone to slipping.
U.S. Pat. No. 7,117,805, Shakelford, Jr., is typical of current designs. It consists of two motors, two top rails and four winders operating in a configuration as previously described. This invention has the disadvantages of other current state-of-the art boat lifts, namely it requires two motors, one of which is on the waterside of the vessel, and it requires a top rail on either side of the vessel, both of which obstruct access to the vessel.
U.S. Pat. No. 7,383,781, Griffin, is typical of current designs. It consists of two motors, two top rails and four winders operating in a configuration as previously described. It has the disadvantages of requiring a top rail and having more than one motor.
U.S. Pat. No. 7,407,150, Bellantoni, provides a means of lifting of boats and large loads, by means of cables and pulleys alone and without submerged cables. It is stabilized against rotation by the arrangement of the pulleys and lines. It allows construction of a boat lift with only one motor, but requires two top rails along the length of the vessel, which impede access to the vessel.
U.S. Pat. No. 7,607,644, Gibb et al., has one top beam and no motors. It is limited to boats of 5,000 pounds or less because it is manually powered.
All the above patents provide for raising and lowering of a large load without cables or structural members passing over the load, but either allow rotation of the load, or have top rails along the side of the load impeding access, or require more than one motor, or have one or more substantive disadvantages such as requiring mechanisms more complex than a pulley and cable system, or restricting the location and type of pulleys and cable fastenings, or depending upon frictional resistance to prevent rotation of the load, or being limited in the weight that they can lift, or otherwise requiring that the parts of the cable and one or more pulleys to be permanently submerged. Current commercially available boat lifts, although stable against rotations, usually have two motors and two top rails to carry power to four lifting points.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those skilled in the art upon a reading of the specification and a study of the drawings.